The invention proceeds from a lamp, in particular a high-pressure discharge lamp, in accordance with the preamble of claim 1. In particular, these are metal halide lamps, sodium high-pressure lamps or halogen incandescent lamps with a pinch at one end and a ceramic base, but also incandescent lamps with a conventional screw base.
EP-A 261 722 has already disclosed a high-pressure discharge lamp in which the base is fastened on the outer bulb by means of cement. This technique is expensive in terms of time, energy and material, because the cement must be heated up laboriously and, in addition, an inner part (steel strip) is used for inductive heating. In addition, problems arise with high loading, because cracks can arise in the cement, and regions of the cement can harden to different degrees. Finally, it has emerged that when these lamps are installed in special luminaires such high temperature loadings can occur that conditions resembling tropical ones with which the conventional cement cannot cope can occur. Moreover, it has emerged that the conventional cement reacts sensitively to extreme environmental conditions. For example, it tends to corrode in the case of air containing salt or sulfur.
On the other hand, there are known (EP-A 668 639) for incandescent lamps which are operated at low voltage, lamps with a mechanical base in the case of which a metal spring is snapped onto the broad side of the pinch. Reliable holding is performed here by latching the metal spring into a projection or a depression on the pinch.
It is the object of the present invention to provide a lamp in accordance with the preamble of claim 1 which can be produced simply and quickly and stands high loadings.
This object is achieved by means of the characterizing features of claim 1. Particularly advantageous refinements are to be found in the dependent claims.
Basically, the lamp according to the invention has a bulb made from glass which has a longitudinal axis and, as a rule, is sealed at one end by a pinch. This bulb is frequently the outer bulb of a discharge lamp or a halogen incandescent lamp which is produced from quartz glass or hard glass. It can also be the sole bulb of a discharge lamp or incandescent lamp.
In the normal case, two supply leads are guided outward on the pinch, and moreover a base is fastened by means of a metal spring (preferably made from spring steel). The base has a trough-shaped holder loosely adapted to the pinch. The metal spring is bent in a U-shaped fashion. It consists of a base part and two limbs which embrace the pinch. The fastening of the base on the bulb is accomplished by a double spring action of the spring. This takes place, in particular, by virtue of the fact that at least one limb (preferably two) is (are) aligned essentially parallel to the longitudinal axis, but simultaneously has a transverse extent accessible to spring forces, the limb being accommodated in the holder of the base, where its transverse extent is limited by the walls of the holder and thus stressed.
The metal spring is a stamped sheet-metal part or the like, which is bent to form a U. During installation, the metal spring is firstly pushed onto the pinch. It holds particularly well there when the bearing surface (mostly on the broad side, but also possibly the narrow side) has a ribbed structure. Moreover, the spacing between the two limbs before installation in the base should be somewhat smaller (in particular approximately 5 to 10%) than the assigned thickness of the pinch. This creates an adequate provisional retention of the metal spring on the pinch.
The greatest transverse extent of the metal spring, that is to say the spacing between the parts of the two limbs which are most widely separated from one another, is at this point in time, that is to say before the mounting of the base, somewhat larger than the assigned width of the holder in the base. The transverse extent of the metal spring is advantageously greater by approximately 5 to 20% than the transverse extent (that is to say, the width) of the holder of the base.
Given conventional dimensions, this corresponds to a transverse extent of the metal spring which is approximately 0.3 to 1 mm larger than the transverse extent of the holder of the base.
A simple implementation of the double spring action consists in that at least one limb is bent inward or outward, preferably in the direction of the transverse extent, in the shape of a channel, in particular with a cross section shaped in the fashion of a V. The stressing of the channel accomplishes the spring action after the mounting of the base. However, the channel can also be rotated by 90xc2x0, that is to say be aligned in the direction of the longitudinal axis.
The metal spring is advantageously constructed such that it has two symmetrical limbs. This halves the spring excursion to be overcome by each limb, as a result of which the material is subjected to less stress. Moreover, a symmetrical placing of the bulb in the base with respect to the longitudinal axis is thereby assured.
In a second embodiment, the limbs are asymmetrical, only one limb being of channel-shaped design, while the second limb is equipped with lateral ends which are rolled in. They serve the purpose of exact lateral fixing.
The base part of the metal spring advantageously runs at a spacing from the supply leads in a central fashion between the two supply leads. This spacing is particularly important in the case of high-pressure discharge lamps, in which a starting voltage of several kV may be required. Consequently, in these cases a particularly narrow base part is to be used, in order to avoid flashovers between a supply lead and the base part. It is favorable if the spacing of the base part from each supply lead is at least 3 mm.
With regard to a high starting voltage, such as is required in the case of immediate restarting, it is also necessary to ensure a minimum spacing of the limbs from the supply leads. This plays a role chiefly in the case of limbs in which a part, chiefly the attachment piece, is approximately as wide as the broad side of the pinch. Here, the lower edge of the attachment piece is preferably spaced at least 2 mm from the supply leads. The point is that a flashover is to be avoided in this direction, as well.
If possible, at least 1 mm of the spacing should be an air gap. This means that the lower edge of the attachment piece does not bear directly against the pinch, but is spaced from the broad side thereof. It is, furthermore, to be taken into account that shock-hazard protection should also be optimized during installation of the lamp for the safety of the customer. This is implemented by virtue of the fact that the free outer end of the limb ends in the holder at least 2 mm below the upper edge of the base.
A very special advantage flows from the technique, presented here, for fastening the base in the case of novel metal halide lamps with a ceramic discharge vessel, and in the case of sodium high-pressure lamps. Since the discharge vessel is sealed here at two ends for technical reasons, these lamps have an unusually large ratio of length to diameter of the outer bulb. Whereas in the case of conventional metal halide lamps this ratio is approximately 2:1 to 3:1, ceramic metal halide lamps (and sodium high-pressure lamps) can reach a ratio of length to diameter of more than 3:1. It has emerged that when these lamps are installed in the associated lamp holder (or luminaire) the tendency to cracks in the pinch of the outer bulb increases substantially when the base is fastened with cement. The cause is that the cement accomplishes a rigid connection between the outer bulb and base. Even a slight vibrational excitation (such as can be produced upon installing the lamp) suffices to load the outer bulb excessively, since in the case of the ratio of more than 3:1 discussed above it has an unfavorable resonance of the natural vibration. When a mechanical base with the metal spring according to the invention is provided, however, no absolutely rigid connection is produced between the base and bulb. Quite oppositely, the vibration is cushioned and damped by the metal spring, with the result that the risk of glass breakages is eliminated. On the other hand, in the case of the previously known provision of a mechanical base, that is to say without the metal spring according to the invention, it was not possible to achieve adequate fixing. Consequently, the optical quality was unsatisfactory upon installation in a reflector. This problem has now been solved.